This invention relates to optical communications and more particularly to optical transmission systems capable of multiplexing on a single mode fiber optical signals from two types of optical sources.
It has recently been shown that light emitting diode (LED) devices can be used for data transmission over single mode fibers. LEDs which characteristically have wide linewidths advantageously are relatively inexpensive and highly reliable and therefore are likely in the near future to find widespread use for purposes of local data distribution in the telephone plant. Lasers which characteristically have narrow linewidths and are capable of supporting higher bit rate more advanced data services than LEDs are at present still relatively expensive and not as reliable as LEDs. In time, as the cost of laser devices decreases and reliability increases, upgrade of those fiber systems employing LEDs may be desired to provide a subscriber with additional data services that may then be available. Such an upgrade, however, will not likely involve the replacement of the existing LED terminal devices, but rather the addition of laser devices to provide the desired extra services. A transmission system capable of simultaneously transmitting on a single mode filter digital signals from both a wide linewidth LED source and a narrow linewidth laser source is therefore desirable. Wavelength-division multiplexing techniques are well known for multiplexing multiple optical signals on a fiber by using different wavelengths for each transmitted signal. Since the spectrum of the LED source is so wide, however, wavelength division multiplexing techniques can not be readily used to multiplex an LED signal with another optical signal. Therefore a system that can multiplex a wide linewidth signal and a narrow linewidth signal is needed so that efficient use of installed optical transmission facilities can be made.